Does your Halloween costume include decorative contact lenses? If so, the Food and Drug Administration has a warning for you: Make sure you get a prescription from an optometrist or other eye care professional.

Eyes that are red, purple, cat-like or otherwise creepy can certainly enhance a costume, but what should really scare consumers are some of the health risks of wearing cosmetic lenses purchased at a Halloween store or other non-medical setting, the FDA says. Risks include scratching or infecting the cornea (the clear, outer layer of the eye); conjunctivitis (also known as pink eye); impaired vision; and even blindness, according to the agency.

“The problem isn’t with the decorative contact lenses themselves,” Dr. James Saviola, an optometrist and FDA eye safety expert, explains in the video above. “It’s the way people use them improperly.”

Contact lenses – even ones that aren’t designed to correct vision problems – are considered medical devices and are subject to FDA regulation, Saviola says. And despite the way they are sold in costume shops, they aren’t one-size-fits-all.

In order to use decorative lenses safely, you must get your eyes measured by an optometrist, ophthalmologist or other eye care professional and get a prescription for a specific type of lens, Saviola says. If you must buy your lenses from a non-medical vendor, make sure they at least require a prescription, he adds.

Under no circumstances should consumers buy lenses from Halloween or other novelty stores, beauty supply stores, salons, street vendors, flea markets, convenience stores or boutiques, the FDA warns. These types of vendors are not authorized by the FDA to distribute contact lenses.

Once you’ve obtained your lenses through an authorized dealer, don’t forget to clean and disinfect them just as you would regular corrective contact lenses. Otherwise you’ll run the risk of infection.

“Bacterial infections can be extremely rapid, result in corneal ulcers, and cause blindness — sometimes within as little as 24 hours if not diagnosed and treated promptly,” Dr. Bernard Lepri, an FDA optometrist, explains in this “Consumer Update.”

If you fail to follow these steps (or even if you do) and wind up with eye redness, persistent pain or vision problems, see a doctor right away, the FDA advises.